In paper machine concepts, the on-line process is becoming increasingly more important both in new machines and when old paper machines are modified. In on-line finishing, in particular in on-line multi-nip calendering of demanding grades, totally new demands are set for the profiles of paper, in particular for moisture and tension profiles, especially in connection with small-scale control of the moisture and tension profiles. New profile control methods are needed for these in order that the quality of paper and runnability should be on a satisfactory level, for example, in multi-nip calendering. In this respect, it is particularly difficult to control the narrow-scale moisture profile, for which no technically advantageous or economical methods are known in the state of the art. The narrow-scale variations of the moisture profile occurring in wide machines are particularly problematic.
Narrow-scale profile variations are either stationary or momentarily wandering in the width direction of the web. Narrow-scale variations are caused, for example, by the condition of a press felt, which may become striped upon ageing, and by colour stripes on a cylinder surface in a dryer section. In addition, basis weight variations may cause narrow-scale moisture profile variation. Distinguishing between wide-scale and narrow-scale profile variation is, of course, a relative conception and varies with different paper grades; in the manufacture of board, a profile variation of, for example, 100 mm may be regarded as narrow and that of 300 mm as wide, in the manufacture of printing paper, a profile variation of, for example, 50 mm may in turn be regarded as narrow and that of 150 mm as wide, and in the case of sanitary tissues, a profile variation of, for example, 25 mm may be regarded as narrow and that of 75 mm as wide. In this description, by narrow-scale profile variation is primarily meant a profile variation of less than 100 mm, even less than 25 mm in width, and in the profiling of the coating quantity, a profile variation of even less than 10 mm in width.
In the prior art, various arrangements are known for controlling the moisture profile of paper, but these are applicable to profiling of the entire area across the width of the web and to wide-scale control of the moisture profile of the paper web. In the prior art, a moisturizing beam is known in which regulating nozzles are used and by means of which the wide-scale variation of the moisture profile of the paper web in the width direction thereof is controlled. With respect to the prior art relating to moisturizing of the web, reference is made to U.S. Pat. Nos. 3,948,721 and 5,286,348.
The narrow-scale variations of the moisture profile cannot be controlled by the methods known in the state of the art, for example, by means of a moisturizing beam, a steam box, an infrared device, a headbox, a deflection-compensated roll, because their profiling width is too great to control narrow-scale moisture profile variation. In addition, they are suitable for use primarily across the entire width of the web.
As known from the prior art, attempts have also been made to control the problems described above such that the paper web has been overdried, thereby eliminating the narrow-scale peaks occurring in moisture profile variation. This is very costly, because it has not been possible, for example, to directly calender the overdried web but it has been necessary to re-moisturize the web by a plane moisturizer before calendering, It shall be noted in particular that it is difficult to remove moisture at the final stage of drying, overdrying thus being rather expensive and difficult to accomplish.
One problem area in the state of the art has also been the profiling of the coating quantity, the local increase of which has presented a particular problem. As known in the state of the art, it has been possible to locally reduce the quantity of coating, for example, by changing the rod or blade load.